Servers sending clients music?
No.
I see 2 big issues with this.
- High quality music is often very big. We don't want our already laggy connections being bogged down by the transmitting of potentially gigabytes of data.
- Copyright. While this probably shouldn't be a concern for the games devs, its probably not a good idea to build a platform that can transfer music to people when they dont have the rights to do so.
As much as I hate to admit it, he's right. I wouldn't be against a way to customize BGM on the client side of things, but having the servers do it is a no-no.
As for client-side customization it is possible. For example, I used to play multiplayer on Battlefield: Vietnam, the jeeps and some other vehicles had radios you could play, normally it was the standard collection of 70's classic rock but there was a folder for placing your own tracks to play in there. So yeah, I had fun kamikaze-ing Commies in an Army Jeep that was blaring various techno and J-rock tracks. Of course the music was local to your end, anyone else in the jeep with you would hear whatever 70's music was supposed to play at that point.
So yeah, just come up with a few standard tracks for whatever's supposed to be going on, but let players swap those out with their own music collections if they so choose.